JP7697481B2 - Method for producing Na-containing oxide, and Na-containing oxide - Google Patents

Description

This application discloses a method for producing a Na-containing oxide and a Na-containing oxide.

Patent Document 1 discloses a Na-containing oxide having a P2 type structure and a chemical composition represented by Na _x Fe _y Mn _1-y O ₂ (x is less than 1, and y is 1/3 or more and less than 2/3).

JP 2012-201588 A

In Na-containing oxides with a P2-type structure, the O3 phase is likely to form along with the P2 phase. A new technology capable of reducing the O3 phase in Na-containing oxides with a P2-type structure is needed.

The present application discloses the following aspects as means for solving the above problems.
<Aspect 1>
A method for producing a Na-containing oxide having a P2 type structure, comprising the steps of:
Obtaining a precursor comprising at least one element of Mn, Ni and Co;
Coating the surface of the precursor with a Na source to obtain a composite; and
The composite is fired to obtain a Na-containing oxide having a P2 type structure.
Including,
The sintering atmosphere of the composite contains 50% by volume or more of oxygen.
Manufacturing method.
<Aspect 2>
The method of manufacturing aspect 1,
The precursor is a spherical particle,
the composite is obtained by covering 40% by area or more of the surface of the precursor with the Na source, and the Na-containing oxide having a P2 type structure is a spherical particle.
Manufacturing method.
<Aspect 3>
The method for producing aspect 1 or 2,
obtaining a precipitate as the precursor by a coprecipitation method using an ion source capable of forming a precipitate in an aqueous solution together with transition metal ions and a transition metal compound containing at least one element selected from Mn, Ni, and Co;
Manufacturing method.
<Aspect 4>
The method for producing any one of Aspects 1 to 3,
The Na-containing oxide having a P2 type structure has a chemical composition represented by Na _a Mn _x-p Ni _y-q Co _z-r M _p+q+r O ₂ (wherein 0<a≦1.00, x+y+z=1, and 0≦p+q+r<0.17, and the element M is at least one selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W).
Manufacturing method.
<Aspect 5>
A Na-containing oxide,
Constituent elements include at least one element selected from Mn, Ni, and Co, Na, and O;
It has a P2 type structure,
When an X-ray diffraction pattern is obtained for the Na-containing oxide using CuKα as a radiation source, a ratio I _P2 /I _O3 of a diffraction peak intensity I _P2 derived from a P2 type structure to a diffraction peak intensity I _O3 derived from an O3 type structure in the X-ray diffraction pattern is 1.0 or more and 20.0 or less,
The particles are spherical.
Na-containing oxide.

The method disclosed herein can reduce the O3 phase when producing a Na-containing oxide having a P2 type structure.

1 shows an example of a flow of a method for producing a Na-containing oxide having a P2 type structure. 1 is a SEM photograph of the appearance of a Na-containing oxide according to Example 1. 1 is a SEM photograph of the appearance of a Na-containing oxide according to a comparative example. 1 shows the X-ray diffraction patterns of the Na-containing oxides of Examples 1 to 3 and the Comparative Example. 4 shows the relationship between the oxygen concentration during firing and the peak intensity ratio I _P2 /I _O3 .

1. Method for Producing a Na-containing Oxide Having a P2-type Structure As shown in Fig. 1, a method for producing a Na-containing oxide having a P2-type structure according to one embodiment includes obtaining a precursor containing at least one element selected from the group consisting of Mn, Ni, and Co (S1), coating the surface of the precursor with a Na source to obtain a composite (S2), and calcining the composite to obtain a Na-containing oxide having a P2-type structure (S3). Here, the calcination atmosphere for the composite contains 50% by volume or more of oxygen.

1.1 S1
In S1, a precursor containing at least one element of Mn, Ni, and Co is obtained. The precursor may contain at least Mn, one or both of Ni and Co, or may contain at least Mn, Ni, and Co. The precursor may be a salt containing at least one element of Mn, Ni, and Co. For example, the precursor may be at least one of carbonate, sulfate, nitrate, and acetate. Alternatively, the precursor may be a compound other than a salt. For example, the precursor may be a hydroxide. The precursor may be a hydrate. The precursor may be a combination of multiple types of compounds. The precursor may have various shapes. For example, the precursor may be particulate, or may be spherical particles as described below. The particle size of the particles made of the precursor is not particularly limited.

In S1, a precipitate as the precursor may be obtained by a coprecipitation method using an ion source capable of forming a precipitate in an aqueous solution with transition metal ions and a transition metal compound containing at least one element selected from Mn, Ni, and Co. This makes it easier to obtain spherical particles as the precursor. The "ion source capable of forming a precipitate in an aqueous solution with transition metal ions" may be at least one selected from, for example, sodium salts such as sodium carbonate and sodium nitrate, sodium hydroxide, and sodium oxide. The transition metal compound may be the above salt or hydroxide containing at least one element selected from Mn, Ni, and Co. Specifically, in S1, the ion source and the transition metal compound may be made into solutions, and then each solution may be dropped and mixed to obtain a precipitate as the precursor. In this case, for example, water is used as the solvent. In this case, various sodium compounds may be used as the base, and an aqueous ammonia solution or the like may be added to adjust the basicity. In the case of the coprecipitation method, for example, an aqueous solution of a transition metal compound and an aqueous solution of sodium carbonate are prepared, and each aqueous solution is dropped and mixed to obtain a precipitate as the precursor. Alternatively, the precursor can be obtained by the sol-gel method. In particular, the coprecipitation method makes it easy to obtain spherical particles as the precursor.

As described above, the precursor may be a spherical particle. In the present application, the term "spherical particle" refers to a particle having a circularity of 0.80 or more. The circularity of the particle may be 0.81 or more, 0.82 or more, 0.83 or more, 0.84 or more, 0.85 or more, 0.86 or more, 0.87 or more, 0.88 or more, 0.89 or more, or 0.90 or more. The circularity of the particle is defined as 4πS/ ^L2 . Here, S is the orthogonal projected area of the particle, and L is the perimeter of the orthogonal projected image of the particle. The circularity of the particle can be determined by observing the appearance of the particle with a scanning electron microscope (SEM), a transmission electron microscope (TEM), or an optical microscope. When the particle is composed of a plurality of particles, the circularity is measured as an average value, for example, as follows.

(1) First, the particle size distribution of the particles is measured. Specifically, the particle size at 10% cumulative value (D10) and the particle size at 90% cumulative value (D90) in the volume-based particle size distribution are determined by a laser diffraction/scattering method.
(2) Regarding the appearance of the particles whose particle size distribution has been measured, images are observed using a SEM, a TEM, or an optical microscope, and from the particles contained in the images, 100 particles having a circle equivalent diameter (the diameter of a circle having the same area as the orthogonal projected area of the particle) of not less than D10 and not more than D90 determined in (1) are randomly selected.
(3) The circularity of each of the 100 extracted particles is determined by image processing, and the average value is regarded as the "circularity of the particle."

In S1, the precursor may contain element M. Element M is at least one selected from B, Mg, Al, K, Ca, Ti, V, Cr, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W. These elements M have, for example, a function of stabilizing the P2 type structure. The method of obtaining a precursor containing element M is not particularly limited. When obtaining a precursor by coprecipitation in S1, for example, an aqueous solution of a transition metal compound containing at least one of Mn, Ni, and Co, an aqueous solution of sodium carbonate, and an aqueous solution of a compound of element M are prepared, and each aqueous solution is dropped and mixed to obtain a precursor containing element M together with at least one element of Mn, Ni, and Co. Alternatively, in the manufacturing method disclosed herein, element M may not be added in S1, and element M may be doped when Na-doped baking is performed in S2 and S3 described later.

1.2 S2
In S2, the surface of the precursor obtained in S1 is coated with a Na source to obtain a composite. The Na source may be a salt containing Na, such as a carbonate or a nitrate, or may be a compound other than a salt, such as sodium oxide or sodium hydroxide. In S2, the amount of the Na source coated on the surface of the precursor may be determined taking into account the amount of Na lost during the subsequent calcination.

In S2, the coverage of the Na source on the surface of the precursor is not particularly limited. For example, in S2, the above-mentioned composite may be obtained by covering 40 area% or more, 50 area% or more, 60 area% or more, or 70 area% or more of the surface of the above-mentioned precursor with the Na source. Here, when the precursor obtained by S1 is a spherical particle, and the composite obtained by S2 is obtained by covering 40 area% or more of the surface of the precursor with the Na source, the Na-containing oxide having the P2 type structure is likely to become a spherical particle in S3 described later. If the coverage of the Na source is small, when the composite is fired, the P2 type crystal is likely to grow abnormally on the surface of the composite, and the Na-containing oxide is likely to become a plate-like. If the coverage of the Na source is large, when the composite is fired, the crystallites of the P2 type crystal are likely to become small, and the Na-containing oxide is likely to become a spherical particle corresponding to the shape of the precursor.

In S2, the method for coating the surface of the precursor with the Na source is not particularly limited. As described above, when 40% or more of the surface area of the precursor is to be coated with the Na source, various methods can be used. For example, a rolling fluidized coating method or a spray drying method can be used. That is, a coating solution in which a Na source is dissolved is prepared, and the coating solution is brought into contact with the surface of the precursor, or the precursor is dried at the same time as the coating solution is brought into contact with the surface of the precursor. By adjusting the coating conditions (temperature, time, number of times, etc.), 40% or more of the surface area of the precursor can be coated with the Na source.

In S2, the precursor may be coated with an M source together with the Na source. For example, in S2, the precursor obtained by S1, the Na source, and an M source containing at least one element M selected from B, Mg, Al, K, Ca, Ti, V, Cr, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W may be mixed to obtain a composite. The M source may be, for example, a salt containing the element M, such as a carbonate or sulfate, or a compound other than a salt, such as an oxide or hydroxide. The amount of the M source relative to the precursor may be determined according to the chemical composition of the Na-containing oxide after firing.

1.3 S3
In S3, the composite obtained in S2 is fired to obtain a Na-containing oxide having a P2 type structure. Here, the firing atmosphere of the composite contains 50% by volume or more of oxygen. The "firing atmosphere" refers to the atmosphere in the "main firing" for generating the P2 phase. When pre-firing is performed before the main firing, the atmosphere in the pre-firing may or may not contain 50% by volume or more of oxygen. When the atmospheres in both the pre-firing and the main firing contain 50% by volume or more of oxygen, a higher effect is likely to be obtained.

In S3, the composite may be formed as desired, optionally pre-fired, and then fired. The pre-fire of the composite may be performed at a temperature equal to or lower than that of the firing. For example, the pre-fire may be performed at a temperature lower than 700°C. The pre-fire time is not particularly limited. The pre-fire atmosphere is also not particularly limited. The pre-fire atmosphere may be the same as or different from the firing atmosphere. The pre-fire atmosphere may contain 50% or more by volume of oxygen.

In S3, the composite may be sintered at a temperature of, for example, 700°C or higher and 1100°C or lower. It is preferably 800°C or higher and 1000°C or lower. If the sintering temperature is too low, Na doping will not occur, and if the sintering temperature is too high, O3 phase, etc., will likely form instead of P2 phase. There are no particular limitations on the heating conditions from the pre-sintering temperature to the sintering temperature.

The firing time is not particularly limited, and may be, for example, 30 minutes or more and 48 hours or less. However, the shape of the Na-containing oxide can be controlled by the firing time. As described above, in the method of the present disclosure, when the coverage rate of the Na source in the composite is 40 area % or more, when the composite is fired, small P2 type crystals are likely to be formed on the surface. In the method of the present disclosure, the shape of the Na-containing oxide corresponds to the shape of the precursor by growing the P2 type crystal along the surface of the particle so as to connect one P2 type crystallite with another P2 type crystallite. For example, when the precursor is a spherical particle, the Na-containing oxide can also be a spherical particle. If the firing time is too short, Na doping is not performed and the desired P2 type structure is not obtained. On the other hand, if the firing time is too long, the P2 type structure grows excessively, resulting in plate-like particles rather than spherical ones. As far as the present inventor has confirmed, when the firing time is 30 minutes or more and 3 hours or less, spherical particles of the Na-containing oxide are likely to be obtained. The Na-containing oxide obtained after this firing may have a structure in which multiple crystallites are present on the surface and the crystallites are interconnected.

As described above, in S3, the firing atmosphere in at least the main firing contains 50% or more by volume of oxygen. According to the inventor's new findings, when the main firing is performed in an atmosphere in which the oxygen concentration is below 50% by volume, such as the air atmosphere, the O3 phase is likely to occur. In contrast, by increasing the oxygen concentration during firing to 50% by volume or more, it is possible to reduce the O3 phase while properly generating the P2 phase. Although the detailed mechanism is unclear, it is believed that when the oxygen concentration during firing is low, some kind of third phase is generated, and this third phase becomes the O3 phase. In S3, the oxygen concentration during firing is 50% by volume or more, which suppresses the generation of this third phase, and as a result, it is believed that the O3 phase is less likely to occur.

2. Na-containing oxide having a P2 type structure By the above-mentioned method, a Na-containing oxide having a P2 type structure and reduced O3 phase can be produced. Hereinafter, a Na-containing oxide according to one embodiment will be described.

2.1 Chemical Composition The Na-containing oxide according to one embodiment contains, as constituent elements, at least one element selected from among Mn, Ni, and Co, Na, and O. In particular, when the constituent elements contain at least Na, Mn, one or both of Ni and Co, and O, and particularly when the constituent elements contain at least Na, Mn, Ni, Co, and O, higher performance is likely to be obtained. In one embodiment, the Na-containing oxide having a P2 type structure may have a chemical composition represented by Na _a Mn _x-p Ni _y-q Co _z-r M _p+q+r O ₂ (wherein 0<a≦1.00, x+y+z=1, and 0≦p+q+r<0.17, and the element M is at least one selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W). When the Na-containing oxide has such a chemical composition, the P2 type structure is easily maintained. In the above chemical composition, a may be more than 0, 0.10 or more, 0.20 or more, 0.30 or more, 0.40 or more, 0.50 or more, or 0.60 or more, and may be 1.00 or less, 0.90 or less, 0.80 or less, or 0.70 or less. Also, x may be 0 or more, 0.10 or more, 0.20 or more, 0.30 or more, 0.40 or more, or 0.50 or more, and may be 1.00 or less, 0.90 or less, 0.80 or less, 0.70 or less, 0.60 or less, or 0.50 or less. Also, y may be 0 or more, 0.10 or more, or 0.20 or more, and may be 1.00 or less, 0.90 or less, 0.80 or less, 0.70 or less, 0.60 or less, 0.50 or less, 0.40 or less, 0.30 or less, or 0.20 or less. In addition, z may be 0 or more, 0.10 or more, 0.20 or more, or 0.30 or more, and may be 1.00 or less, 0.90 or less, 0.80 or less, 0.70 or less, 0.60 or less, 0.50 or less, 0.40 or less, or 0.30 or less. The element M has a small contribution to charging and discharging. In this respect, in the above chemical composition, by p + q + r being less than 0.17, a high charging and discharging capacity is easily ensured. p + q + r may be 0.16 or less, 0.15 or less, 0.14 or less, 0.13 or less, 0.12 or less, 0.11 or less, or 0.10 or less. On the other hand, by including the element M, the P2 type structure is easily stabilized. In the above chemical composition, p+q+r is 0 or more, and may be 0.01 or more, 0.02 or more, 0.03 or more, 0.04 or more, 0.05 or more, 0.06 or more, 0.07 or more, 0.08 or more, 0.09 or more, or 0.10 or more. The composition of O is approximately 2, but is not necessarily exactly 2.0 and is indefinite.

2.2 Crystal structure The Na-containing oxide according to one embodiment has at least a P2 type structure (belonging to space group P63mc) as a crystal structure. The Na-containing oxide may have a P2 type structure and a crystal structure other than the P2 type structure. Examples of crystal structures other than the P2 type structure include various crystal structures formed when Na is deintercalated from the P2 type structure. Here, the Na-containing oxide according to this embodiment has a P2 type structure, while having a small O3 type structure. The Na-containing oxide according to this embodiment may have a P2 type structure as a main phase. For example, when an X-ray diffraction pattern is obtained for the Na-containing oxide according to this embodiment using CuKα as a radiation source, the ratio I P2 /I O3 of the diffraction peak intensity I _P2 derived from the P2 type structure and the diffraction peak intensity I _O3 derived from the O3 type structure in the X- _{ray diffraction} pattern may be 1.0 or more, 2.0 or more, 3.0 or more, or 4.0 or more. The upper limit of I _P2 /I _O3 is not particularly limited, and may be, for example, 20.0 or less, 18.0 or less, 16.0 or less, 14.0 or less, or 12.0 or less.

The diffraction peak intensity I _P2 due to the P2 type structure is determined as follows.
(1) In an X-ray diffraction pattern using CuKα as a radiation source, the average value _I35.0 of the diffraction peak intensity at 35.0°±0.2° is determined as the background.
(2) In the X-ray diffraction pattern, the maximum diffraction peak intensity _I39.85 at 39.85°±0.1° is determined. The diffraction peak at 39.85°±0.1° corresponds to the (102) plane of the P2 type structure.
(3) The value obtained by subtracting I _35.0 from I _39.85 is the above-mentioned I _P2 (I _P2 = I _39.85 - I _35.0 ).

The diffraction peak intensity I _O3 due to the O3 type structure is determined as follows.
(1) In an X-ray diffraction pattern using CuKα as a radiation source, the average value _I35.0 of the diffraction peak intensity at 35.0°±0.2° is determined as the background.
(2) In the X-ray diffraction pattern, the maximum diffraction peak intensity _I38.0 at 38.0°±0.2° is determined. The diffraction peak at 38.0°±0.1° corresponds to the (012) plane of the O3 type structure.
(3) The value obtained by subtracting I _35.0 from I _38.0 is defined as the above I _O3 (I _O3 = I _38.0 - I _35.0 ).

The Na-containing oxide according to one embodiment may be a single crystal consisting of one crystallite, or may be a polycrystal having multiple crystallites. For example, the surface of the Na-containing oxide according to one embodiment may be composed of multiple crystallites. In other words, the Na-containing oxide may have a structure in which multiple crystallites are connected to each other on its surface. When the surface of the Na-containing oxide is composed of multiple crystallites, a crystal grain boundary is present on the surface. Here, the crystal grain boundary may be an inlet and an outlet of intercalation. That is, when the Na-containing oxide is a polycrystal having multiple crystallites, the effect of increasing the number of inlets and outlets for intercalation to reduce the reaction resistance, the effect of shortening the distance traveled by sodium ions to reduce the diffusion resistance, the effect of reducing the absolute amount of expansion and contraction during charging and discharging, and the effect of making it difficult for cracks to occur, etc. can be expected. The size of the crystallite may be large or small, but it is considered that the smaller the size of the crystallite, the more the crystal grain boundaries will be, and the more likely the above-mentioned advantageous effects will be exhibited. For example, if the diameter of the crystallite constituting the Na-containing oxide is less than 1 μm, higher performance is likely to be obtained. The "crystallite" and "diameter of the crystallite" can be determined by observing the surface of the Na-containing oxide with a scanning electron microscope (SEM) or a transmission electron microscope (TEM). That is, when the surface of the Na-containing oxide is observed and a closed region surrounded by a grain boundary is observed, the region is regarded as a "crystallite". The maximum Feret diameter of the crystallite is determined and regarded as the "diameter of the crystallite". If the Na-containing oxide is composed of a single crystal, the particle itself can be said to be a single crystallite, and the maximum Feret diameter of the particle is the "diameter of the crystallite". Alternatively, the diameter of the crystallite can be determined by EBSD or XRD. For example, the diameter of the crystallite can be determined based on the Scherrer formula from the half-width of the diffraction line of the XRD pattern. If the diameter of the crystallite of the Na-containing oxide specified by any method is less than 1 μm, the performance of the Na-containing oxide is likely to be higher.

2.3 Shape The P2 type structure is a hexagonal crystal system, has a large diffusion coefficient of Na ions, and is likely to grow crystals in a specific direction. In particular, when at least one of Mn, Ni, and Co is included as a transition metal element constituting the P2 type structure, the crystals tend to grow in a plate-like shape in a specific direction. Therefore, the Na-containing transition metal oxide having the P2 type structure usually becomes a plate-like particle with a large aspect ratio in which the crystal growth direction is biased in a specific direction. In contrast, the Na-containing oxide according to one embodiment may be a spherical particle as described above. When the Na-containing oxide is a spherical particle, as described above, the reaction resistance is reduced by reducing the crystallite size, and the diffusion resistance inside the particle is likely to be reduced. Furthermore, when applied to a secondary battery or the like, it is considered that the degree of bending is reduced by spheroidization, and the sodium ion conduction resistance is reduced. As a result, for example, the rate characteristic is improved and the reversible capacity is likely to be increased.

The Na-containing oxide according to one embodiment may be a solid particle, a hollow particle, or a particle having a void. The size of the Na-containing oxide particles is not particularly limited, but it is considered that a smaller size is more advantageous. For example, the average particle diameter (D50) of the Na-containing oxide particles may be 0.1 μm or more and 10 μm or less, 1.0 μm or more and 8.0 μm or less, or 2.0 μm or more and 6.0 μm or less. The average particle diameter (D50) is the particle diameter (D50, median diameter) at an integrated value of 50% in the volume-based particle size distribution by the laser diffraction/scattering method.

2.4 Supplementary Note In summary, the Na-containing oxide according to one embodiment may have, for example, the following configurations (1) to (4).
(1) The sodium-containing oxide contains, as constituent elements, at least one element selected from the group consisting of manganese, nickel, and cobalt, sodium, and oxygen.
(2) The Na-containing oxide has a P2 type structure.
(3) When an X-ray diffraction pattern is obtained for the Na-containing oxide using a CuKα radiation source, the ratio I _P2 /I _O3 of the diffraction peak intensity I _P2 attributable to a P2 type structure to the diffraction peak intensity I _O3 attributable to an O3 type structure in the X-ray diffraction pattern is 1.0 or more and 20.0 or less.
(4) The Na-containing oxide is a spherical particle.

3. Uses The Na-containing oxide having a P2 type structure produced by the above method can be used, for example, as a positive electrode active material of a sodium ion battery. A sodium ion battery according to one embodiment includes a positive electrode active material layer, an electrolyte layer, and a negative electrode active material layer, and is characterized in that the positive electrode active material layer contains the Na-containing oxide having the P2 type structure of the present disclosure as a positive electrode active material. The sodium ion battery can have the same configuration as a conventional battery, except that it contains the specific positive electrode active material.

As described above, one embodiment of the method for producing a Na-containing oxide having a P2 type structure has been described, but the production method of the present disclosure can be modified in various ways other than the above embodiment without departing from the gist of the method. Below, the technology of the present disclosure will be described in more detail while showing examples, but the technology of the present disclosure is not limited to the following examples.

1. Preparation of Na-containing oxide having P2 type structure 1.1 Preparation of precursor (1) _MnSO4.5H2O , _NiSO4.6H2O , and _CoSO4.7H2O were weighed out to obtain the desired composition ratio, and dissolved in distilled water to obtain a concentration of _1.4 mol/ _L to obtain a first solution. In a separate container _{, Na2CO3} was dissolved in distilled water to obtain a _{concentration} of 1.4 mol/L to obtain a second solution.
(2) 800 mL of pure water was placed in a reaction vessel (with a baffle plate), and 500 mL of the first solution and 500 mL of the second solution were each added dropwise at a rate of about 4 mL/min.
(3) After the dropwise addition was completed, the mixture was stirred at room temperature at a stirring speed of 150 rpm for 1 hour to obtain a product.
(4) The product was washed with pure water, subjected to solid-liquid separation using a centrifuge, and the precipitate was collected.
(5) The obtained precipitate was dried overnight at 120° C., pulverized in a mortar, and then fine particles were removed by air classification to obtain precursor particles. The precursor particles were a composite salt containing Mn, Ni, and Co, and were spherical particles having a circularity of 0.80 or more.

1.2 Preparation of Composite (1) Na ₂ CO ₃ as a Na source and the above-mentioned precursor particles were weighed out so that the composition after firing described below would be Na _0.7 Mn _0.5 Ni _0.2 Co _0.3 O ₂ .
(2) The weighed Na source and precursor particles were mixed by spray drying. Specifically, the weighed Na source and precursor particles were added to a solvent, and the dispersion solution in which the Na source was dissolved and the precursor particles were dispersed was spray dried. The spray drying temperature was 200° C., and the spray pressure was 0.3 MPa. By spray drying, a composite in which 75 area % of the surface of the precursor particles was covered with the Na source was obtained.

1.3 Firing of the Composite 6 g of the composite was placed in an alumina crucible and fired. The firing atmosphere was air for the comparative example, 92% oxygen for Example 1, 75% oxygen for Example 2, and 50% oxygen for Example 3. In Examples 1 to 3, gas replacement was performed in the system at the start of firing, and the oxygen concentration was changed by changing the mixture ratio of pure oxygen and air. In addition, oxygen gas was flowed at 0.05 L/min during firing. The conditions in the firing process are as follows (1) to (7).
(1) An alumina crucible containing the above composite is placed in a heating furnace.
(2) The temperature inside the heating furnace is raised from room temperature to 600° C. in two hours.
(3) The inside of the heating furnace is kept at 600° C. for 2 hours to perform pre-firing.
(4) After the preliminary firing, the temperature in the heating furnace is increased from 600° C. to 900° C. over a period of two hours.
(5) The inside of the heating furnace is kept at 900° C. for one hour to carry out main firing.
(6) After the main firing, the temperature in the heating furnace is lowered from 900° C. to 250° C. over a period of 4 hours.
(7) Remove the alumina crucible from the heating furnace at 250° C. and allow it to cool in the air.

The fired product after cooling in _air was pulverized in a mortar in a dry atmosphere to obtain Na-containing oxide particles having _{a P2} type structure. The Na-containing oxide particles had a chemical composition represented _{by Na0.7Mn0.5Ni0.2Co0.3O2} .

2. Evaluation of Na-containing oxide particles 2.1 Appearance observation by SEM FIG. 2 shows an appearance SEM photograph of the Na-containing oxide according to Example 1. FIG. 3 shows an appearance SEM photograph of the Na-containing oxide according to the comparative example. As is clear from FIGS. 2 and 3, the Na-containing oxide according to Example 1 and the comparative example are spherical particles having a circularity of 0.80 or more. It can also be seen that the surface of the spherical particles is composed of a plurality of crystallites, and the crystallite diameter is less than 1 μm. The Na-containing oxides according to Examples 2 and 3 also had the same form as Example 1.

2.2 Identification of crystal structure by X-ray diffraction measurement For each of the Na-containing oxides of Examples 1 to 3 and Comparative Example, X-ray diffraction measurement was performed using CuKα as a radiation source to obtain an X-ray diffraction pattern, and the ratio I _P2 /I _O3 of the diffraction peak intensity I _P2 derived from the P2 type structure to the diffraction peak intensity I _O3 derived from the O3 type structure was obtained from the X-ray diffraction pattern. FIG. 4 shows the X-ray diffraction patterns of each of the Na-containing oxides of Examples 1 to 3 and Comparative Example. FIG. 5 shows the relationship between the oxygen concentration during firing and the peak intensity ratio I _P2 /I _O3 . As shown in FIGS. 4 and 5, the Na-containing oxide according to the Comparative Example had a ratio I _P2 /I _O3 of the diffraction peak intensity I _P2 derived from the P2 type structure to the diffraction peak intensity I _O3 derived from the O3 type structure of less than 1.0, that is, it contained a large amount of O3 phase. In contrast, the Na-containing oxides according to Examples 1 to 3 had a ratio I _P2 /I _O3 of 1.0 or more, and the amount of the O3 phase was significantly reduced compared to the comparative example. In Examples 1 to 3, it is believed that the generation of the O3 phase was suppressed by the oxygen concentration during main firing being 50 volume % or more.

3. Supplementary Note: In the above examples, the precursor is obtained by coprecipitation, but the precursor can be obtained by other methods. In the above examples, the surface of the precursor is coated with a Na source by spray drying to obtain a composite, but the composite can be obtained by other methods. In the above examples, the Na-containing oxide having a P2 type structure is exemplified as having a predetermined chemical composition, but the chemical composition of the Na-containing oxide is not limited to this. Various chemical compositions having a P2 type structure can be adopted. In addition, the Na-containing oxide may be doped with an element M other than Mn, Ni, and Co. The element M is as described in the embodiment. In the above examples, the spherical precursor is used to finally obtain spherical Na-containing oxide particles, but the precursor and the Na-containing oxide are not limited to spherical particles. However, in the case of spherical particles, for example, a high effect can be expected as a positive electrode active material for a battery.

As described above, the method for producing a Na-containing oxide having a P2 type structure, which includes the following steps S1 to S3, can suppress the generation of the O3 phase in the Na-containing oxide. That is, it is possible to produce a Na-containing oxide having a P2 type structure and reduced O3 phase.
S1: A precursor containing at least one element of Mn, Ni, and Co is obtained.
S2: The surface of the precursor is coated with a Na source to obtain a composite.
S3: The composite is fired to obtain a Na-containing oxide having a P2 type structure, wherein the firing atmosphere of the composite contains 50% by volume or more of oxygen.

According to the above-mentioned manufacturing method, for example, a Na-containing oxide that satisfies the following properties (1) to (4) can be obtained.
(1) The alloy contains at least one element selected from the group consisting of Mn, Ni, and Co, as well as Na and O as constituent elements.
(2) It has a P2 type structure.
(3) When an X-ray diffraction pattern is obtained using CuKα as a radiation source, the ratio I _P2 /I O3 of the diffraction peak intensity I P2 attributable to the P2 type structure to the diffraction peak intensity I _O3 attributable to the O3 type structure in the X- _{ray diffraction} pattern is 1.0 or more and 20.0 or less.
(4) The particles are spherical.

Claims (4)

A method for producing a Na-containing oxide having a P2 type structure, comprising the steps of:
Obtaining a precursor comprising at least one element of Mn, Ni and Co;
Coating the surface of the precursor with a Na source to obtain a composite; and
The composite is fired to obtain a Na-containing oxide having a P2 type structure.
Including,
The sintering atmosphere of the composite contains 50% by volume or more of oxygen.
Manufacturing method. The method according to claim 1,
The precursor is a spherical particle,
the composite is obtained by covering 40% by area or more of the surface of the precursor with the Na source, and the Na-containing oxide having a P2 type structure is a spherical particle.
Manufacturing method. The method according to claim 2,
obtaining a precipitate as the precursor by a coprecipitation method using an ion source capable of forming a precipitate in an aqueous solution together with transition metal ions and a transition metal compound containing at least one element selected from Mn, Ni, and Co;
Manufacturing method. The method according to any one of claims 1 to 3,
The Na-containing oxide having a P2 type structure has a chemical composition represented by Na _a Mn _x-p Ni _y-q Co _z-r M _p+q+r O ₂ (wherein 0<a≦1.00, x+y+z=1, and 0≦p+q+r<0.17, and the element M is at least one selected from B, Mg, Al, K, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, and W).
Manufacturing method.

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